1. Field of the Invention
The present invention relates to systems for irradiating samples at cryogenic temperatures, more particularly to X-ray crystallography systems, and even more particularly to goniometer bases used in X-ray crystallography systems.
2. Description of the Related Art
X-ray crystallography is a primary method for determining the molecular structure of inorganic compounds, organic compounds (including those of pharmaceutical relevance), proteins, nucleic acids and viruses. In this method, a sample (usually a crystal) to be examined is held in an X-ray beam, and the X-rays diffracted by the sample are measured using a detector. The sample orientation is changed in a precisely controlled manner, usually by rotating it about one or more axes using a motor-driven goniometer stage, and diffraction data is collected at multiple orientations. These data from different orientations are then merged and analyzed to determine the molecular structure. Because most crystals are easily damaged by the X-ray beams, X-ray data is typically collected on frozen samples, with a cold (T=100 K) gas stream flowing over the sample during measurements. Herein, the term “cryogenic temperatures” will be used to mean temperatures below T=180 K, low enough to perform X-ray crystallography without excessive sample damage due to X-rays. It is noted that the sample studied in X-ray “crystallography” does not always necessarily have a crystal structure.
Most small molecule and protein/biomolecule crystallography is now performed using a standard set of tools. A crystal is retrieved using a tool (hereinafter, “crystal holding tool”) consisting of an X-ray transparent loop or tip attached to a small (typically 0.64 mm (0.025 inch) diameter, 19 mm long) steel rod (hereinafter, “crystal holding tool rod” or “rod”). These crystal mounting tools are currently sold under trade names such as CryoLoop (Hampton Research, Aliso Viejo, Calif.), MicroMount and MicroLoop (MiTeGen, LLC, Ithaca, N.Y.), and LithoLoop (Molecular Dimensions). (Note: the terms “CryoLoop,” “MicroMount,” “LithoLoop” and/or “MicroLoop” may be subject to trademark rights in various jurisdictions throughout the world and are used here only in reference to such trademarked products or services to the extent that such trademark rights may exist.)
The rods of these tools are inserted into goniometer bases (hereinafter referred to as “goniometer bases” or “bases”) or “caps” (so-called because the first bases were derived from the caps of cryogenic vials), and then glued into the bases using epoxy or other adhesive to firmly secure them. In other words, the rod receiving hardware set in these conventional goniometer bases is made up of some type of recess for receiving the rod and epoxy for rigidly mechanically connecting to the goniometer base. This rigid connection formed by the hardened epoxy prevents the rod from moving with respect to the goniometer base. Conventionally, it is believed that this rigid connection is a desirable feature because it prevents the rod from moving around, relative to the precisely controlled goniometer stage and to the X-ray beam, when the rod assembly and the sample it holds are subjected to cryogenic gas flows and/or cryogenic fluid flows. This rigid connection is also a desirable feature to prevent the rod from moving relative to the base, including falling out of the goniometer base, due to forces during goniometer rotation and in other routine handling.
These bases are then attached to the rotating goniometer stage assembly. The goniometer stage assembly generally contains a magnet-containing goniometer head that holds and orients the goniometer base, which holds and orients the crystal holding tool rod and rod-receiving hardware set, which holds and orients the sample in the X-ray beam at a precisely controlled position and angular orientation in three-space.
FIG. 1A shows goniometer base and crystal holding tool assembly 100 including: sample holding loop 102; rod 104; and goniometer base housing 108. Housing 108 encloses a rod receiving hardware set (not shown, generally including a rod recess and epoxy) and a stage connection hardware set (not shown, for mechanically connecting the goniometer base to the goniometer stage assembly) In one tool set that is particularly well-suited to high-throughput automated sample handling, housing 108 consists of two parts: a magnetic stainless steel base member 110 and a copper upper part 106, as shown in FIG. 1A, FIG. 2C (goniometer base 220), and FIG. 2D (goniometer base 230). It is copper upper part 106 that contains the rod receiving hardware set 107, which is basically made up of epoxy and a hole, as shown in FIG. 1B. In other goniometer base assemblies the base includes only a single magnetic stainless steel part, as shown in FIG. 2A (goniometer base 200) and FIG. 2B (goniometer base 210), that contains the rod receiving hardware set and also any necessary hardware for attaching to the goniometer stage assembly. The bottom of the base is secured to a magnet-containing goniometer head (not shown) that rotates and translates to position the sample in the X-ray beam. In the other common tool set (used almost exclusively in small molecule crystallography), a single piece brass or stainless steel base 240 as shown in FIG. 2E is held in a hole in the goniometer head using a set screw. Goniometer bases 200, 210, 220, 230 (respectively shown in FIGS. 2A-2D) are for use with magnetic goniometer heads. Herein, goniometer bases for use with magnetic goniometers will be specifically referred to as magnetic goniometer bases.
There are several shortcomings with the current technology, in use for nearly 20 years, recognized by those skilled in the art. All current commercial goniometer bases simply have cylindrical holes into which the crystal holding tool rod is inserted, thus the bases do not positively grip the rods to keep the crystal holding tool rod from falling out of the base during handling (which can cause automated handlers to fail). The conventional thinking is that this problem is best overcome by gluing the rod into the base, most commonly using an epoxy.
Because of the sub-millimeter dimensions of the crystal holding tool rod, this gluing of rods into bases is difficult and time consuming, especially in the quantities of hundreds to thousands required in modern high-throughout crystallography. The vertical positioning of the crystal holding tool in the base (i.e., the distance between the bottom of the base and the crystal-holding aperture) is fixed once the glue sets and cannot later be adjusted to optimize the crystal's position in the X-ray beam. When the tip 102 of the crystal holding tool becomes damaged (a common occurrence), both the crystal holding tool and base are typically discarded, rather than incurring the time and expense involved in removing the old glue and regluing.
Nylon loop crystal holding tools (CryoLoops) are rapidly being replaced with higher performance but more fragile microfabricated polymer film tools, which must be periodically replaced.
However, the above-noted drawbacks of rigid connections between goniometer bases, and the rods that they hold, are currently generally considered as acceptable because of the degree of securement and positional stability provided to the rod.